1. Field of the Invention
The present invention relates to the tight sealing of vertical shafts, particularly rotary driven shafts of the type used in hydraulic machines such as turbines.
2. History of the Invention
The seal between the vertical rotary driven shaft of such machines and the fixed parts adjacent thereto raises delicate problems. A defective or loose seal brings about disastrous economic consequences from the standpoint both of repairs and of down time of the machine.
When the shaft of the hydraulic turbine is vertical, the seal adapted to ensure a tight seal between the fixed and mobile parts of the turbine is constituted by an annular wear plate or ring associated with the rotating part of the turbine which is abutted on opposite sides by continuous annular seal elements of rectangular section, generally made of carbon. These annular seal elements are mounted on a fixed ring which is movable axially with respect to the shaft. This ring which constitutes the seal-holder is loaded or urged into abutting engagement with the wear plate by a series of compression springs distributed equally about the geometric axis of the shaft of the machine. In addition, there exists an annular chamber which is filled with liquid under pressure. The liquid, which is generally clean water, is directed from the chamber into the space between the sealing elements so as to lubricate and cool them. This water is at a pressure higher than that prevailing in the enclosure of the wheel of the turbine, so that there is a permanent leakage in the direction of the enclosure.
Known seals present considerable drawbacks. In fact, when a turbine-like machine is shut off, the springs associated with the seals apply a constant force which tends to urge the annular seal elements against the wear part or plate. This presure is even greater when the machine is shut off and the supply of spray water is accidentally cut off. Under these conditions, when the machine is started up again, there is very frequently a deterioration of the seals created by the increases in frictional forces when the seal elements are dry. It will be readily understood that this drawback is very serious since replacement of seal elements involves a complete dismantling of all the parts of the machine located above the wear plate.
When the turbine is shut off, the dirty water which surrounds the turbine wheel may penetrate between the two seals, if, for example the outer one, is scratched. The result is that the dirty water passes into the circular chamber where it stagnates resulting in impurities being deposited therein which impurities or deposits often damage the seals of the circular chamber. If this is the case, the seals must be changed and, as their diameters are of the order of two meters, the difficulty and cost of such an operation will be readily appreciated.